Ore body

“ The manner in which metallic substances are distributed through the earth's crust, and more especially the phenomena of those nearly vertical and tabular masses of ore called mineral veins, from which the larger part of the precious metals used by man are obtained,—these are subjects of the highest practical importance to the miner, and of no less theoretical interest to the geologist. ”

“ In iron ore, the metallic oxide or carbonate makes up nearly the whole rock; in gold ore, the metal is the merest incidental constituent, from the petrologic point of view.
Concentration.—The essential fact in the formation of ores is the unusual concentration of the metal. There are vast quantities of all the metals disseminated through the rock substance of the earth and even throughout the hydrosphere, but they do not constitute ores because they have no economic value. They become ores when concentrated in accessible places to a workable richness. ”

“ Seldom, however, do the processes go far enough in nature to produce an ore which can be used directly without some further mechanical sorting. Ore minerals concentrated in placers are those which resist abrasion and chemical solution during the processes of weathering and transportation, and which have a density sufficiently high so that they are partially sorted out and concentrated from the accompanying quartz and other minerals. ”

“ A very common way of locating an ore-mine is by the nature of the out-crop,—that is, the broken edges of strata of rocks protruding from hillsides, or tilted at an angle on level areas. If the ore-bearing vein is harder than the surrounding strata it will be found as a jutting edge, protruding beyond the surface of the other layers of rocks which, being softer, are more easily worn away. ”

“ The production of ore which occurs in the upper parts of the veins, furnishes us with the saline ores; for here the water, air and earth are principally active. In the upper depths are found most frequently the oxygenized metals, or as they are called mineral spars, as spathic-iron, malachite, calamine, lead-spar, pyromorphite, &c. ”

“ If, however, plant or animal life be present in sufficient quantities, this oxidation is prevented. In case but little foreign material, clay or sand, has been brought by the waters, the deposit will be an iron ore. In case large quantities of foreign material are deposited from the waters at the same time, there will be produced, in the absence of life, a brown or red clay or sandstone, and in its presence a white or light colored formation containing the iron as a carbonate. ”

“ Rich ore bodies frequently occur on the borders of igneous dikes, and in fissures and cavities in igneous rocks, but the process by which they have been formed is similar to that leading to the concentration of mineral matter in metamorphic rocks, and will be referred to later. ”

“ To these facts may be added the presence of thick beds and veins of iron ore and of apatite or calcium phosphate (bone earth) . Both of these substances occur in a disseminated state in nearly all rocks, but they are concentrated into definite deposits by the action of life. Iron is usually dissolved out and redeposited by acids produced in the decay of vegetable matter, as we see in the clay ironstones of the coal formation and in bog-iron ores. ”

“ The ores occur not only in veins and shoots, but also in great masses replacing the limestone, and the geologists who have examined these mining districts are nearly unanimous as to the metasomatic nature of a large part of these deposits. Other rocks such as tuff, volcanic breccia, shale, porphyry and granite may also be impregnated with metalliferous ores, but the largest ore bodies are found in the limestones. Secondary enrichment has often taken place on a considerable scale. ”

“ In veins or lodes, it is not supposed that ore-making minerals could have been precipitated from solutions travelling either upward or downward and obeying chemical laws if the depth were sufficient to furnish great temperature or high rock and hydrostatic pressures. ”

“ The ore is found in detached pieces and solid masses, in beds, in red clay, accompanied by sulphate of barytes, calcareous spar, blende, iron pyrites, and quartz. The ore is of that kind called, by mineralogists, lead-glance, or galena, and is the sulphuret of lead, of chemistry. As it is dug up or quarried from the adhering spar, it presents a very rich appearance. It has a [Pg 171] broad, glittering grain, of a lead-gray colour, which passes into a bluish shade. The ore is easily broken by the blow of a hammer, and may be pounded to a fine powder, still preserving its glittering appearance. ”

“ Perhaps no other portion of the earth's surface is so rich in mineral resources as that which has been covered by these observations, and nowhere else is to be found as great a variety of ore deposits, or those which illustrate as well their mode of formation. This is so true that it maybe said without exaggeration that no one can intelligently discuss the questions that have been raised in regard to the origin and mode of formation of ore bodies without transversing and studying the great mining belt of our Western States and Territories. ”

“ An ore formed by direct processes of sedimentation has sometimes been called primary, whereas an ore formed by later enrichment of these sediments has been called secondary. An ore formed directly by igneous processes has been called primary, while an ore formed by enrichment of such primary ore by later processes has been called secondary. ”

“ OF OPAKE OBJECTS.
Ores and minerals afford an immense variety of very beautiful and splendid objects. From amongst these the observer may select the peacock or coloured copper ore, green crystallized ditto, [609] lead ore, crystallized ditto, crystals of lead, small grained marcasites, coloured mundic, cinnabar, native sulphur, needle and other antimony, moss copper, &c. A mixture of small pieces of ores, &c. of different kinds, produces a pleasing effect. Sands in general exhibit something not discoverable with the naked eye. ”

“ The leaner ores contain a great deal of calcareous matter in the shape of common limestone or spar, which reduces the percentage in the ore as low as between 15 and 25 per cent., and it seldom exceeds 25, except when mixed with fragments of what is called brush ore, which, when in quantity, raises the percentage to 40 or 45. ”

“ It is the centre of a district very rich in minerals, obtained from a narrow stretch of crystalline schists underlying the Tertiary deposits. Iron ores (60 to 70% of iron) , copper ores, colours, brown coal, graphite, slate, and lithographic stone are obtained—nearly 2,000,000 tons of iron ore annually. ”

“ These ores have some sulphur in them, and are therefore baked in ovens to remove it. They yield about 50 per cent. of iron. A geologist some time ago reported upon the ore banks that there were thirty millions of tons of ore in sight above the water-level, being over three times the amount taken out since the workings began in the eighteenth century. The deposits extend to a depth of several hundred feet under the surface, thus indefinitely multiplying the prospective yield. ”

“ Where an acid substance, like silica, is balanced by basic constituents like lime and magnesia, considerable amounts of both may be used. Excessive moisture content may spoil an ore because of the amount of heat necessary to eliminate it in smelting. ”

“ Iron ore, tin, copper and tungsten are extracted from the earth, refined, put to some use or wasted as the case may be, but they are gone. They may be replaced by other minerals. Through geological ages they may redeposited in the earth's crust. ”

“ Replacement implies the precipitation of the ore and gangue, molecule by molecule, in the position of the original minerals but without, as in pseudomorphs, the necessary reproduction of crystalline forms. Some waterway must of course introduce the ore-bearing solutions, but it may be slight compared with the great size of the resulting ore-bodies. Lead and zinc ores, often carrying some silver, are those most widely distributed, as they were also the earliest recognized in deposits of this character. More than any other metals their association with limestone is pronounced. ”

“ Iron ores are seldom so uniform in quality that they can be shipped without careful attention to sampling and grade. In the Lake Superior region the ores are sampled daily as mined, and the utmost care is taken to mix and load the ore in such a way that the desired grades can be obtained. Ordinarily a single deposit produces several grades of ore. When ores are put into the furnace for smelting the mixtures are selected with great care for the particular purpose for which the product is to be used. ”

“ The ore used is the common sulphuret, with a foliated, glittering and cubical fracture. It occurs with scarcely any adhering gangue. Cubical masses of it are found, at some of the diggings, which are studded over with minute crystals of calcareous spar. These crystals, when examined, have the form of the dog-tooth spar. This broad, square-shaped, and square-broken mineral, is taken from east and west leads, is most easy to smelt, and yields the greatest per centum of lead. It is estimated to produce fifty per cent. ”

“ Lead ores occur in deposits of several distinct genetic types. Many deposits lie at shallow depth in sedimentary rocks, without apparent connection with igneous rocks, occurring as tabular replacements of receptive beds. In regions of slightly disturbed strata the ore shoots tend to follow pitching troughs. ”

“ In alluvial districts we find ores of clay-ironstone, granular iron-ore, bog-ore, swamp-ore, and meadow-ore. The iron ores which belong to the primitive period have almost always the metallic aspect, with a richness amounting even to 80 per cent. of iron, while the ores in the posterior formations become in general more and more earthy, down to those in alluvial soils, some of which present the appearance of a common stone, and afford not more than 20 per cent. of metal, though its quality is often excellent. ”

“ The ore (the author remarks) is the lead glance, galena, or sulphuret of lead. It is very rich and beautiful, and specimens in our possession fully confirm Mr. Schoolcraft's account; they have a very broad and perfectly foliated fracture, and a high degree of metallic lustre; they break in cubical fragments, and the minutest portions still retain this form.
We have already observed that large fragments are found loose in the earth: they sometimes weigh four or five pounds; we have such specimens from these mines ”

“ You see, the ore in the mine contains radium, that is, salts of radium. It is a pitchblende deposit, and it happens to be so rich in radium content that throughout the ages it has affected all the life in the cavern. The arid land surrounding the ore—this has been, generally, one of the characteristics of radium deposits—has kept most of the jungle creatures away, but underground beings such as reptiles, worms and frogs, have gradually become immune to the effects of the ore and have grown prodigiously and abnormally under the stimulation of the rays given off by the radium. ”

“ In like manner, through205 their hydrogenous import, they pass over into the inflammable asphalts, and through these unto sulphur. Metal and sulphur have, in the Geogeny, announced themselves as the precursors, or harbingers, of the vegetable world. In this respect, also, can the vegetable kingdom be regarded as the mineral kingdom, that, having continued to grow, has become alive. The ore, which becomes organic, becomes carbon or plant. ”

“ The air itself causes rocks to crumble; percolating water robs them of their soluble salts, reducing even solid granite to a loose mass of quartz grains and clay. Plants and animals absorb as food the mineral substances of rocks, when they are dissolved in water. They transform these food elements into their own body substance, and finally give back their dead bodies, the mineral substances of which are freed by decay to return to the earth, and become elements of rock again. ”

“ Only in rare cases does the ore itself form a mass which fuses at the temperature employed, and these cases are objectionable if much iron oxide is carried away in the slag. The impurities of the ores most often consist of certain mixtures—for instance, a mixture of clay and sand, or a mixture of limestone and clay, or quartz, &c. ”

“ All along the Appalachian Mountains the iron has been gathered in beds which are being mined. These beds of ore are usually mixed with clay and other earthy substances from which the metal can be separated only by melting. ”

“ Certain varieties of iron ore deposits are now being built up by the aid of plants, and similar ores in the ancient rocks may have had a like origin. The presence of great quantities of carbon, in the form of graphite, may be regarded also as a sign of life, for this substance is accumulated on a large scale by living plants, and may be retained in a solid form after the partial decay of the plant tissues. ”

“ The reason of this is, that the strata of rocks which lie beneath the ground in all this region consist, in a great measure, of beds of coal and of iron ore. The miners dig down in almost any spot, and find iron ore ”

“ The result would be carbonates and sulphides of metals, earths, alkalies, and free quartz, but quartz [43] and sulphides of the metals are soluble in solutions of carbonates and sulphides of the earths and alkalies, and the essential constituents of the ore might, therefore, readily be conveyed to openings in the vein where they would have been deposited on relief of pressure and diminution of temperature. ”